This invention relates to a system, apparatus, and method for regenerating an electroless bath electrolyte.
In addition to galvanic plating methods in which an external current is introduced via electrodes that are placed into the plating bath for the purpose of depositing a metal plating on a work piece, so-called electroless plating methods are known. These methods are most often used to plate nonconducting substances, for example, plastic components. Such metal platings, for example, are applied for various reasons to plastic substrates. For one thing, a metal surface may be desired for esthetic reasons; for another, the objective may be to utilize the material properties of the metal with which a substrate is plated. Such properties include, for example, improved resistance to corrosion or the electrical conductivity of the material used. Thus, it is known, for example, that printed conductors can be applied to printed circuit boards made of plastic (for example, epoxy resins) by means of electroless plating techniques.
In particular, nickel metal is frequently deposited by means of electroless plating technology.
To reduce the metal ions contained in the electrolyte to elemental metal when this type of plating technology is used, an appropriate reducing agent which is oxidized itself during the reaction must be added to the electrolyte. In the case of an electroless nickel bath, hypophosphite ions are added. These reduce the nickel ions contained in the precipitation bath to elemental nickel and are themselves oxidized to orthophosphite ions. The equation of the reaction that takes place when an electroless nickel bath is used can be expressed as follows:
NiSO4+6NaH2PO2(copyright) Ni+2H2+2P+4NaH2PO3+Na2SO4
In the course of the metal plating process, nickel ions are gradually removed from the electrolyte and are precipitated as elemental nickel on the surface to be plated; at the same time, hypophosphite ions are continuously oxidized to orthophosphite ions. In other words: for one thing, the concentration of the nickel ions dissolved in the electrolyte and the concentration of the hypophosphite ions contained in the electrolyte decrease in the electrolyte, and for another, the concentration of the orthophosphite ions contained in the electrolyte increases. As a result, the electrolyte is being xe2x80x9cdepleted.xe2x80x9d Thus, as the time during which the electrolyte is allowed to stand increases, the quality of a plating deposited with such an electrolyte decreases. This means that the electrolyte can be used only for a certain number of plating runs. Thereafter, the electrolyte must either be replaced or it must be regenerated by means of suitable auxiliary agents. For the nickel precipitation bath, regeneration involves at least the removal of the orthophosphite ions which form as reaction products and, potentially, the addition of spent nickel ions and spent hypophosphite ions.
In addition to the in situ precipitation of undesirable ions in sparingly soluble compounds and the addition of ions which are needed and which are spent in the course of the standing time of the bath, it is also known that electrodialytic methods can be used for the regeneration of electroless precipitation baths. Such methods provide that the depleted bath electrolyte and a regeneration electrolyte which absorbs at least the ions which are to be removed from the depleted bath electrolyte so as to be able to regenerate [the bath electrolyte] are channeled through compartments which are separated from each other by membranes. At the same time, the regeneration electrolyte may contain ions that are to be added to the bath electrolyte. Via electrodes provided in an electrodialysis unit, current is conducted through an electrodialysis unit and an ionic flow is induced. Through the suitable selection of the membranes located between the so-called diluate compartments through which the bath electrolyte flows and the so-called concentrate compartments through which the regeneration electrolyte flows, it is possible to ensure a targeted migration of ions from the bath electrolyte which is passing through the diluate compartments into the regeneration electrolyte which is passing through the concentrate compartments and vice versa.
An example of such an electrodialysis system is described in the German Patent No. DE 198 49 278 C1. In the system described in this printed publication, two separate electrodialysis units are used, each of which comprises diluate compartments and concentrate compartments that are separated from one another by membranes as well as a pair of electrodes, i.e., an anode and a cathode. On the cathode side, the diluate compartments of a first electrodialysis unit are separated from the concentrate compartments of this unit by monoselective cation exchanger membranes and on the anode side by anion exchanger membranes. In the second electrodialysis unit which also comprises diluate compartments and concentrate compartments as well as an anode and a cathode, the diluate compartments are separated from the concentrate compartments by monoselective anion exchanger membranes on the cathode side and by anion exchanger membranes on the anode side. To regenerate the bath electrolyte, this electrolyte is divided into two main streams which are conducted parallel to each other through the diluate compartments of the first electrodialysis unit and the second electrodialysis unit. Similarly, the regeneration electrolyte is divided into substreams which are conducted parallel to each other through the concentrate compartments of the first and the second electrodialysis unit. In the first electrodialysis unit, both orthophosphite ions and hypophosphite ions are removed from the bath electrolyte. Nickel ions still present in the bath electrolyte remain in the electrolyte. In the diluate compartments of the second electrodialysis unit, hypophosphite ions from the regeneration electrolyte are fed into the second substream of the bath electrolyte.
This method is less efficient per run, and it is necessary to recirculate the bath electrolyte to be regenerated several times through the electrodialysis system until the degree of regeneration desired is obtained.
A second system known from prior art is disclosed in EP 0 787 829 A1. This electrodialysis system described in this printed publication also comprises two electrodialysis units which have both diluate compartments and concentrate compartments as well as an anode and a cathode. As to its setup, the first electrodialysis unit of this printed publication is similar to the electrodialysis unit of the German patent specification mentioned above. Again, the diluate compartments of the first electrodialysis unit are separated from the adjacent concentrate compartments by a monovalent cation exchanger membrane on the cathode side and by an anion exchanger membrane on the anode side. The second electrodialysis unit of this electrodialysis system, however, has a setup different from the setup known from the German patent specification above. In this case, the diluate compartments on the cathode side are separated from the adjacent concentrate compartments by a cation exchanger membrane and on the anode side by a monovalent anion exchanger membrane. In the system that is known from this printed publication, both the regeneration electrolyte and the bath electrolyte flow sequentially in one direction through the individual electrodialysis units. In the first electrodialysis unit, the bath electrolyte is depleted of hypo- and orthophosphite ions, and in the second electrodialysis unit, hypophosphite ions are returned in a second step. Thus, the system known from EP 0 787 829 A1 is the starting point for the present invention as disclosed in the precharacterizing clause of main claim 1.
The system known from the European patent application [Offenlegungsschrift], however, has the disadvantage that its setup is expensive and that the electrodes used in the electrodialysis units are not sufficiently protected against the detrimental influences of the chemicals contained in the electrolytes.
Thus, using this well-known prior art as a starting point, the problem to be solved by the present invention is to further develop an electrodialysis system of the type described in the introduction by designing it so that it is less expensive to construct and by considerably increasing the life of the electrodes used.
This problem is solved according to the present invention by providing that the electrodes have separate electrode compartments which are separated from the adjacent compartments by membranes and through which a rinsing electrode is channeled via third lines and that an electrode common to and functioning for both electrodialysis units is located in one of the electrode compartments which are adjacent to compartments of both electrodialysis units.
Thus, the setup of the system for the electrodialytic regeneration of an electroless bath electrolyte according to the present invention has the advantage that two electrodialysis units, through which a sequential flow takes place, jointly utilize one electrode, which means that only three electrodes are used to construct the two electrodialysis units. The jointly utilized electrode may be an anode or a cathode. The use of only three electrodes in the setup of the system according to the present invention makes the use of an otherwise required fourth electrode superfluous and thus reduces the cost of manufacturing such a system. In addition, the overall system which comprises separated electrodialysis units can now be constructed so as to be more compact and space-saving. Between the concentrate and diluate compartments of the first dialysis unit which are adjacent to each other and the diluate and concentrate compartments of the second electrodialysis unit which are adjacent to each other, an electrode compartment housing the electrode jointly utilized by both electrodialysis units is inserted. The number of diluate and concentrate compartments per electrodialysis unit is not limited but can instead be adjusted, as needed, to the throughput of the bath electrolyte to be purified. The key factor is to ensure that, after passing through the first electrodialysis unit, the bath electrolyte passing through the diluate compartments of the first electrodialysis unit passes through the diluate compartments of the second electrodialysis unit.
By incorporating electrode compartments which are separated from the concentrate and diluate compartments and through which a separate rinsing electrolyte can flow, it is ensured that the electrodes are shielded from the ions dissolved in the bath electrolyte and in the regeneration electrolyte so that said ions cannot have a detrimental effect on the electrodes. Instead, the electrode compartments are rinsed with a rinsing electrolyte which ensures, on the one hand, that current can flow from the electrode compartments into the concentrate and diluate compartments of each electrodialysis unit and, on the other hand, that the life or standing time of the electrodes used is considerably increased.
According to a useful further development of the present invention, it is proposed that the rinsing electrolytes present in the electrode compartments be sodium sulfate, potassium sulfate, or sodium phosphate. According to another useful further development of this invention, these [rinsing electrolytes] are used in a concentration ranging from 1 g/L to 30 g/L. A rinsing electrolyte of the composition proposed has good conducting properties, but the concentration of the dissolved ions is not yet high enough to damage the membranes and the electrodes. In addition, the electrolyte has a viscosity sufficiently high for pumping the electrolyte.
In another useful embodiment of the present invention, it is proposed that the system for channeling the bath electrolyte and/or the regeneration electrolyte into the diluate and concentrate compartments of at least one electrodialysis unit have parallel lines to the separate diluate and concentrate compartments, which parallel lines originate from a main feeder line. To channel the electrolytic streams of the bath electrolyte and/or the regeneration electrolyte through an electrodialysis unit, each stream is divided into substreams and channeled parallel to each other through a number of diluate compartments and concentrate compartments of this unit. After the substreams have been channeled through the individual compartments, they are recombined to form one bath electrolyte and one regeneration electrolyte and as such are separately channeled into the second electrodialysis unit or a collecting tank for further use. By channeling the electrolyte streams in the form of substreams parallel to one another through several diluate and concentrate compartments of an electrodialysis unit, the throughput can be increased. The effective ion exchange capacity between a diluate compartment and an adjacent concentrate compartment is multiplied by the number of the diluate and concentrate compartments used.
According to another useful further development of the present invention, a closed loop line for channeling the bath electrolyte through the electrodialysis system is proposed. For this purpose, it is useful to provide for a collecting tank in which the bath electrolyte to be regenerated is stored and from which it is taken and to which it is returned after having been regenerated in the electrodialysis system. The tank can be a bath tank in which the electroless plating is carried out. In such a configuration, the bath electrolyte is so-to-speak regenerated in situ in that a certain quantity of the bath electrolyte contained in the plating tank is removed and regenerated by means of the regeneration system. The quantity of bath electrolyte thus regenerated is channeled back into the plating tank where it recombined with the residual electrolyte. Depending on the requirements that the bath electrolyte must meet, the regeneration rate required can be controlled via the volumetric rate of flow of the bath electrolyte per unit time through the electrodialysis system. If the bath electrolyte used in the process needs to meet higher xe2x80x9cpurityxe2x80x9d requirements, the volumetric rate of flow per unit time through the electrodialysis system for the same bath tank volume must be higher. Correspondingly, the electrodialysis units of the electrodialysis system must be configured for a higher throughput. For this purpose, they may have, for example, a greater number of diluate and concentrate compartments than would be required for a lower electrolyte throughput.
Since, as a rule, the electrolyte for electroless metal plating is used at an increased operating temperature, it is proposed in another useful further development of the present invention that at least in the feed line of the electrodialysis system, but preferably also in the return flow line, a heat exchanger be provided. By means of the heat exchanger in the feed line, i.e., the supply line to the first electrodialysis unit, of the system, the electrolyte is cooled by means of a cooling medium, for example, cooling water. In this manner, the sensitive components of the electrodialysis unit, such as the membranes, are not damaged by an excessively hot electrolyte. With a heat exchanger that is installed in the return flow line of the electrodialysis system, i.e., in the drainage line for the bath electrolyte from the second electrodialysis unit, the bath electrolyte which now has a temperature below the operating bath temperature is again preheated before it is returned to the collecting tank, in particular the bath tank. The two heat exchangers used can be configured in such a way that the cooling medium which is heated in the course of cooling the bath electrolyte contained in the heat exchanger of the feed line of the system is used to heat the bath electrolyte, which is now cold, in the return flow line of the system.
To prevent particles from penetrating the electrodialysis units, another useful further development of this invention proposes that a filter be placed into the feed line of the system, i.e., into the line feeding the bath electrolyte into the first electrodialysis unit. This filter filters particles from the bath electrolyte which may have precipitated and thus prevents clogging of the sensitive membranes between the individual compartments of the electrodialysis unit. It should be ensured that the size of the particles trapped by the filter is sufficiently small, i.e., the filter should be sufficiently fine. For this purpose, cross-flow filtration (micro- or nanofiltration) among other things can be used.
According to yet another useful further development of the present invention, the system also has a closed loop line for channeling the regeneration electrolyte. As proposed in another useful further development of the present invention, this closed loop line preferably comprises a storage tank, from which the regeneration electrolyte is channeled to the first electrodialysis unit and from which the regeneration electrolyte from the second electrodialysis unit is returned. In the collecting tank, the composition of the regeneration electrolyte can be adjusted specifically to the regeneration requirements. Thus, the orthophosphite ions which were removed as waste products from the bath electrolyte can be removed from the regeneration electrolyte, for example, by precipitation. To adjust to an optimum pH value, acids or bases can be added. In addition, nickel ions which had optionally been added as spent material to the bath electrolyte as well as hypophosphite ions can be added at this point.
According to yet another useful further development of the present invention, it is finally proposed that a closed loop line be also provided for the rinsing electrolyte for the electrode compartments. Again, a collecting tank can preferably be provided for this electrolyte, from which the rinsing electrolyte for the electrode compartments flows sequentially through the individual electrode compartments and into which the rinsing electrolyte returns at the end. Since, as a result of the catalysis of water to hydrogen and oxygen which takes place on the electrodes, water is continuously removed from the rinsing electrolyte, it may be useful, if needed, to add water at this point, i.e., at the collecting tank for the rinsing electrolyte, to this electrolyte. For this purpose, a feed line can be provided.
Other characteristics and advantages of the present invention can be taken from the following description of an embodiment of a system for the electrodialytic regeneration of an electroless nickel electrolyte according to the present invention which is shown in the single attached figure.